It's actually quite hard to say. Ali had more slashing speed, was lighter on his feet and quicker in combination (because he wasn't putting everything into the shots) and seemed to have quicker reactions. Tyson had more explosiveness and more herky jerky speed when attacking an opponent, but wasn't necessarily the most fleet-footed of guys on the backfoot. I'd probably go with Ali, but it has to be tempered with an understanding of their very different styles and the strategies they brought to a fight.
It's a different type of speed, dancing and fluttering jabs quickly vs rushing somebody with head movement and nice angles, and really putting together combinations quickly but with a lot more power, I'd say neither is faster or slower, just used their speed differently. If I had to say, then Mike